The present invention relates to pantograph safety and protection systems and, more particularly, to an emergency lowering system for a pantograph.
Electric powered vehicles such as transit cars, trolleys and locomotives typically are powered from a wayside power source which may comprise an elevated catenary system. Catenary systems are well known and generally include a catenary or messenger cable strung between vertical supports, an electrical line or contact wire for carrying electrical power and a plurality of spaced dropper lines supporting the contact wire from the catenary cable. The dropper lines are connected to the contact wire by metal clips. With this arrangement the contact wire is supported in a substantially uniform horizontal plane by varying the lengths of the dropper lines. At curves the contact wire is pulled into an arc by the use of dropper lines or pull off lines which pull the contact wire in a horizontal direction.
The vehicles employ roof mounted pantographs which extend vertically to engage the current carrying contact wire and supply power to the vehicle. The pantographs typically include an articulated support and a current collector, the latter being referred to as a pan, bow or shoe. The current collector or pan has an elongated flat upper contact surface designed to be maintained in sliding contact with the elevated overhead contact wire. The pan also includes depending arcuate members or horns extending from the opposite ends of the contact surface. The horns minimize the abrupt transition from the contact surface if a contact wire slides laterally off the surface and tends to encourage the contact wire to slide up and onto the contact surface if some counteracting lateral force is present.
The articulated support for the pan is a spring supported arrangement which urges the pan upward with a substantially uniform force, e.g., 26 to 28 pounds. During operation of the vehicle, this spring force is sufficient to maintain the pan in sliding contact with the contact wire without being sufficiently strong to appreciably lift the contact wire itself. The pantograph is lowered by counter-biasing means which may comprise, for example, a pneumatic jack operated by a controllable source of a pressurized gas such as air.
It will be appreciated that if the contact wire does not exert a downward force on the pantograph, the spring biasing system will urge the pantograph into its fully extended position. If, for example, the contact wire is broken, the pantograph will lose contact with the wire and will be urged into a fully extended position. Similarly, if dropper lines become broken, the slack created in the contact wire may allow it to slide laterally off the current collector. In either of these events the fully extended pantograph on a moving vehicle may become tangled in the dropper lines and result in destruction of portions of the catenary system and the pantograph itself. In a train of vehicles having a plurality of pantographs, each pantograph entering the discontinuous section may become fully extended and similarly entangled, it being noted that in a typical train arrangement, so long as one pantograph remains in operative contact with a contact wire, the train will continue to receive power and be propelled.
The problems resulting from loss of contact between the current collector and contact wire have been recognized by the prior art. In U.S. Pat. No. 3,444,338 a protection system is proposed which would sense abnormal stress on the pantograph such as would occur upon impact of the fully extended pantograph with the overhead catenary system. In an alternative embodiment, limit switches on the articulated support are set to detect full extension of the pantograph. In both these systems, detection of the indicated condition will activate a lowering system to retract the pantograph to a rest position. Protective systems of this and other types are used by British Railways and are described in a paper by D. J. W. Souch and G. Taylor entitled Development of the Pantograph For High Speed Collection and published by the Institution of Mechanical Engineers on Mar. 8, 1971. A further system described in the referenced paper includes a piping system which pipes the gas for the pneumatic lowering jack to the underside of the pan through a control valve. In the event of the pan being dislodged the piping will be ruptured and vent to atmosphere whereby the control valve will be operated and force lowering of the pantograph. Each of the above-described systems require that the pantograph become fully extended before corrective action is taken and at least some of the systems require that impact and possible damage occur before the pantograph is lowered.
In studying the problems which result in damage to a pantograph, it has been found that a majority of the problems are caused by an out of position contact wire as a result of dropper line pull off line, or clip failures rather than breakage of the contact wire. Consequently, most problems begin when a contact wire slides off an end of a current collector rather than a current collector running off the end of a contact wire at a break point.